The present invention relates to a method of forming a mask used for the charged particle beam exposure and a program for processing pattern data for forming a mask for the charged particle beam exposure, and more particularly to a method of forming a mask used for the EPL (Electron-beam Projection Lithography) and a program for processing pattern data therefor.
In recent years, accompanying progress in the degree of integration of semiconductor devices, miniaturization of the patterns to be projected into semiconductor wafers have been further advancing, and for the projection of such minute patterns, exposure methods with charged particle beams such as electron beams or ion beams have been employed.
In the exposure methods, using charged particle beams, for example, electron beams, stencil masks are generally utilized as masks for the pattern projection. The stencil mask has a structure in which openings in the form of a prescribed pattern is set in a thin substrate made of silicon or similar material. With such a stencil mask being used, in the electron beam exposure method, an electron beam consisting of electrons which pass through the openings in the form of a prescribed pattern travels through a projection optical system and forms an image on a wafer, and thereby projection of the pattern is made. An electron beam consisting of electrons which, in irradiation, fall on a region of the stencil mask other than the openings is thereby blocked from reaching the wafer by the mask substrate through absorption, reflection or heavy scattering. Having traversed the mask substrate, some rays of the electron beam are heavily scattered and are cut off by a limiting aperture in the projection optical system, and they do not arrive on the wafer.
Meanwhile, in the electron beam exposure method, there is also utilized a membrane-type mask (referred to as a xe2x80x9ccontinuous membrane maskxe2x80x9d hereinafter) in which an electron-beam scatterer in a prescribed shape is set on an electron-beam transmittable thin film with a small electron-beam scattering power. In the electron beam exposure method utilizing such a continuous membrane mask, electrons which traverse the thin film region of the mask where no electron-beam scatterer is formed form an image on a wafer, and thereby projection of the pattern is made. The electrons, having traversed the electron-beam scatterer of the mask, are heavily scattered and cut off by a limiting aperture so that they cannot arrive on the wafer.
As against the continuous membrane mask of this sort, the afore-mentioned stencil mask is widely used. The reason lies in a fact that, with the stencil mask, electrons to form an image on the wafer pass through the opening thereof so that, compared with the continuous membrane mask with which image-forming electrons have to traverse a thin film thereof, no energy loss by the mask occurs or the resolution is not lowered by chromatic aberration, and, thus, a high resolution can be obtained. Moreover, fabrication of stencil masks is also relatively easy.
For the stencil mask of this sort, however, the pattern must be formed by the layout of openings. This gives rise to a problem that the pattern shape possible to be formed becomes limited.
For example, because a structure with a pattern (referred to as a xe2x80x9cdoughnut patternxe2x80x9d hereinafter) in which a screening region to block the electron beam is completely surrounded by an opening, as shown in FIG. 1(a), cannot itself support its screening region that is like an isle surrounded by an opening, and this region will fall out, it is impossible to form a stencil mask with this pattern. Furthermore, in a structure with a pattern (referred to as a xe2x80x9cleaf patternxe2x80x9d hereinafter) wherein an opening is not set around the entire perimeter of the inner screening region but formed in such a way that the inner screening region is linked with the peripheral screening region by a part of its perimeter, as shown in FIG. 1(b) (leaf type) and FIG. 1(c) (tongue type), the linking section has a substantially low mechanical strength so that the inner screening region is bound to fall off, while the mask is in use or even in fabrication. In addition, for a structure with a pattern (referred to as an xe2x80x9cL-shaped patternxe2x80x9d hereinafter) in which an opening is in the shape of the letter xe2x80x9cLxe2x80x9d or the shape that the letters xe2x80x9cLsxe2x80x9d are joined together, the stress is liable to center at both ends of corner sections of the screening region which extends into the opening, and thus the mask may be easily damaged at those sections.
Accordingly, to overcome these problems, there has been employed a method in which a prescribed pattern is divided into a plurality of sections and, by distributing these sections onto two masks, complementary opening patterns are formed in respective masks, and, using these two complementary masks, pattern transcription is carried out twice and, at the end, a prescribed pattern is transcribed onto a wafer (J. Vac. Sci. Technol. B (1993) Vol. 11(6), pp. 2400-2403).
For instance, in Japanese Patent Application Laid-open No. 132206/1994, there is disclosed a method wherein a doughnut pattern shown in FIG. 2(a) is divided into two, and, using alternately a first mask with an opening pattern shown in FIG. 2(b) and a second mask with an opening pattern shown in FIG. 2(c), and a doughnut pattern is transcribed.
Further, in U.S. Pat. No. 5,166,888, a method of splitting a pattern in formation of complementary masks is disclosed. An example of this method is described below, with reference to FIG. 3. In FIG. 3(a), a prescribed pattern (H) that is to be transcribed onto a wafer is shown, and a set of cutting lines for the pattern is each shown in FIGS. 3(a-1) and (a-2). The cutting lines shown in FIG. 3(a-2) are the ones selected to use. FIG. 3(b) shows a stencil mask having an opening in the form of a pattern corresponding to FIG. 3(a). Further, FIGS. 3(b-1) and (b-2) show a set of two complementary masks, which are used alternately to transcribe, at the end, the pattern of FIG. 3(a).
Firstly, all corners of a polygon (100), an outline of which is in the form of the prescribed pattern (H), are determined. Next, all inside corners 102, 103, 104 and 105 are determined. Herein, all corners other than inside corners are called outside corners.
Next, as shown in FIG. 3(a-1), cutting lines 107, 108, 109 and 110 are laid down from each inside corner to its opposing sides. Although cutting lines are, here, laid down from all inside corners, only an inside corner whose specifically assigned stability value does not satisfy a specific criterion can be selected for the inside corners from which cutting lines are actually laid down. Next, among these sets of possible cutting lines, a set of cutting lines which have the shortest lengths and do not intersect one another are chosen (112, 113, 114 and 115 of FIG. 3(a-2)). The prescribed pattern (H) is, then, split by these chosen cutting lines into a plurality of sections (Hxe2x80x2), and a plurality of these sections (Hxe2x80x2) are distributed to two masks. That is, in one mask, as shown in FIG. 3 (b-1), there are formed opening patterns 117, 119 and 120, each of which corresponds to a section (Hxe2x80x2) distributed to this mask, out of a plurality of sections (Hxe2x80x2), while, in the other mask, as shown in FIG. 3(b-2), there are formed opening patterns 118 and 121, each of which corresponds to a section (Hxe2x80x2) distributed to this second mask, out of a plurality of sections (Hxe2x80x2).
Although the above disclosure has been made to propose a splitting technique for the projection of patterns that may bring about a problem to the stencil mask, any effective method capable to extract singular patterns such as a doughnut pattern, a leaf pattern and an L-shaped pattern systematically from a group of various patterns has not been known. For that reason, even a pattern which hardly requires splitting is processed together for pattern splitting. In consequence, not only the processing efficiency becomes low but also numerous minute pattern sections (segments) are produced and, in the pattern projection, exact connection positions for the pattern sections become difficult to find so that changes in dimensions of the pattern connection sections or even breaking thereof may occur, lowering reliability of the pattern projection.
An object of the present invention is to provide a stencil mask by an efficient method, wherein singular patterns which may cause a problem in stencil mask formation are systematically extracted and split, whereby the number of pattern openings is minimized while maintaining sufficient mechanical strength, and the reliability of the pattern projection is raised.
The present invention relates to a method of forming a mask with an opening pattern used for a charged particle beam exposure; which comprises the steps of:
contouring a pattern figure represented by prescribed pattern data;
descerning whether every polygonal pattern represented by pattern data being obtained through said contouring is a convex polygon or not;
splitting every polygonal pattern which is not identified as a convex polygon into a plurality of pattern sections; and
distributing a plurality of said pattern sections onto masks which constitute a set of complementary masks.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set forth above, wherein, in said step of discerning, a signed area of triangle xcex94pixe2x88x921pipi+1 composed of three adjacent vertices pixe2x88x921, pi and pi+1 is obtained one by one for a vertex pi in the polygonal pattern, and once there are identified vertices for which a sign of the area xcex94pixe2x88x921pipi+1 becomes opposite to the sign thereof with which an interior angle ∠pixe2x88x921pipi+1 satisfies ∠pixe2x88x921pipi+1 less than 180xc2x0, it is determined that said polygonal pattern is not a convex polygon.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set forth above, which further comprises a step of:
pattern reversing in which, for every polygonal pattern which is not identified as a convex polygon in said step of discerning, there is set a pattern area for reversing, a boundary of which is composed of four sides of the smallest rectangle containing the whole of the pattern and each side lying parallel to an orthogonal coordinate axis, and by reversing the pattern in this pattern area for reversing, a complementary pattern is formed; wherein
in said step of pattern splitting, in every said pattern area for reversing, a cutting line is laid down from every vertex of said complementary pattern to run parallel to a coordinate axis, and thereby pattern splitting is carried out.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set for the above, wherein, in said step of pattern splitting, in every said pattern area for reversing, from every vertex of said complementary pattern, a cutting line to run parallel to an X-axis and a cutting line to run parallel to a Y-axis are laid down, and when, out of two sets, a set of cutting lines parallel to the X-axis and a set of cutting lines parallel to the Y-axis, one set is to be selected, a set having a larger minimum cutting distance is chosen, and if their minimum cutting distances are the same, a set provided with a smaller number of cutting lines is chosen.
Further, the present invention relates to a method of brining a mask used for the charged particle beam exposure as set forth above, wherein, for every polygonal pattern which is not identified as a convex polygon in said step of discerning, setting a pattern area, a boundary of which is composed of four sides of the smallest rectangle containing the whole of the pattern and each side lying parallel to an orthogonal coordinate axis, and
with the exception of any polygonal pattern having the outline which, in shape, is a polygon containing another polygon inside, with the inside polygon free from internally touching the perimeter of the outside polygon, the polygonal pattern in said pattern area for reversing is not subjected to said pattern splitting, when the length of every side of the pattern area for reversing is less than a prescribed standard value.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set forth above, which further comprises a step of:
pattern reversing in which, for every polygonal pattern which is not identified as a convex polygon in said step of discerning, there is set a pattern area for reversing, a boundary of which is composed of four sides of the smallest rectangle containing the whole of the pattern and each side lying parallel to an orthogonal coordinate axis, and by reversing the pattern in this pattern area for reversing, a complementary pattern is formed; wherein
in said pattern area for reversing having a complementary pattern which has, at least, one side in common with the boundary of the pattern area for reversing and, at least, two sides independent of the pattern area boundary, if, for the mask that is to be fabricate, a force applied onto a line segment joining two vertices on the boundary of said pattern area for reversing (excepting vertices of the pattern area) out of vertices of said complementary pattern, is less than a prescribed standard value, said pattern splitting is not to be applied thereto.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set forth above, wherein, in said step of distributing, one pattern section cut out by said pattern splitting is distributed to a mask different from a mask to which another pattern section, having shared a side on a cutting line therewith, is distributed.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set forth above, wherein, in said step of distribution, a polygonal pattern that is undivided without having received the pattern splitting is distributed to one of the complementary masks so as to equalize, at the end, an area density of pattern figures assigned thereto with the one assigned to every other complementary masks.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set for the above, which further comprises a step of rectangular pattern correction, for a rectangular pattern which is, being one of polygonal patterns, identified as a convex polygon in said step of discerning, and has a rectangular pattern figure, and besides an area of the pattern figure exceeds a prescribed reference area; wherein pattern correction is carried out in such a way that with four corners being cut off within a permissible range of accuracy of dimension for the opening pattern, the original rectangular pattern is made to take the form of an octagon.
Further, the present invention relates to a method of forming a mask used for the charged particle beam exposure as set for the above, which further comprises a step of rectangular pattern splitting for a rectangular pattern which is, being one of polygonal patterns, identified as a convex polygon in said step of discerning, and has a rectangular pattern figure, and besides an area of the pattern figure exceeds a prescribed reference area; wherein the original rectangular pattern is split into an octagon from which four corners have been cut off and four triangles corresponding to four corners which have been cut off; and said octagonal pattern and four triangle patterns are distributed onto different masks.
Further, the present invention relates to a program for processing pattern data to form a mask with an opening pattern used for charged particle beam exposure, which makes a computer carry out every step in the method as set forth above.
Further, a xe2x80x9crectanglexe2x80x9d as used in the present invention includes a xe2x80x9csquarexe2x80x9d as one of its possible form.
In the present invention, any of the singular patterns which may cause a problem in stencil mask formation is, through systematical identification, extracted and then, each of these singular patterns is split and distributed onto different masks constituting a set of complementary masks so that, while maintaining sufficient mechanical strength, the number of openings can be reduced to a minimum by avoiding superfluous pattern splitting. As a result, a stencil mask capable to improve the reliability of the pattern projection can be provided efficiently.